Gravity wave center

In all the years that bars have been observed, I don't think a sustained explanation has been found. The difficulty is what is visible vs what is not. Models based purely on luminous matter distribution run into difficulties, as the observed tangential velocities of stars as a function of radial distance from galactic centre doesn't fall off fast enough. This eventually lead to the concept of 'dark matter' which, by definition, allegedly contributes to such dynamics but is unseen. ( Don't confuse this with 'dark energy' which refers to a quite unrelated idea on a vaster cosmological scale ) Until a full accounting is achieved of all gravitationally active elements then explanations of all structural patterns remain moot.
Cheers, Mike.

I've just looked a few things up.
If we judge matters upon visible stuff alone, then the overall/average tangential velocities of stars in a galaxy should fall off like the inverse square root of the radius from galactic centre. This is very similiar to our own solar system, and is a 'Keplerian' distribution if you like.
However for very many galaxies looked at to date the measured velocities are pretty constant ( ie. not subsiding ) with radius, and out to a considerable distance too. It is posed therefore that a globular distribution of invisible material ( aka dark matter ), also centred on the galactic centre as visibly defined, would explain things. If so one can deduce some properties of said dark matter.
Now some observations have shown that no edge has yet to be defined for the extension of such dark matter into intergalactic space. I think the motions of smaller satellite galaxies similiar to our Magellenic clouds in the Milky Way were considered here. This raises the serious possibility of vast extensions of dark matter into what are otherwise deemed 'voids', many galactic radii away.
Studies of gravitational lensing of light from some galaxy clusters have deduced the presence of considerable non-luminous mass that contributes to the bending of the light. ( The entire material produces multiple images of some other galaxy or object well behind the cluster as viewed by us. By analysing the images carefully one can uniquely determine the mass distribution that produced it ).
Cheers, Mike.

Thanks.
Now that's a BIG sucker! Significantly they've deduced that structure from different types of studies too...
I like this bit :

Surely that's evidence for an alien LIGO? Complete with end and corner stations.... :-)
Cheers, Mike.

( edit ) IF the Great Attractor/Wall/Region was plainly visible without any special equipment or process, THEN imagine what our cosmology/myth would be like! It spans a considerable slab of sky....

Ernie, Mike:

Actually you can say a lot about galactic structure without knowing where the dark matter is. The spiral arms in spiral galaxies are density waves propagating around the galactic disk due to its rotation. They're not all that different from hurricanes in some ways, but on a grand scale. Instead of a little air getting squeezed so that water droplets condense on a timescale of hours and produce an "arm" of clouds, you've got the interstellar gas getting squeezed so that it condenses on a timescale of millions of years and produces stars. The spiral arm propagates because the biggest stars burn fast and die in big explosions, which push out bubbles in the interstellar medium and compress the next bit of gas further around the galactic disk, which makes more stars down there, which blow ... and so on.

There is some thought that spiral galaxies are just ellipticals that got these waves triggered by close encounters or mergers with other galaxies. Our galaxy is surrounded by several little satellite galaxies (including the Magellanic Clouds which you can see in the southern sky) and a shredded streamer of stars which looks like another one in the process of being eaten. So it looks like we had a "recent" (less than a billion years) encounter with another galaxy that got ripped up and gave us our spirals.

If I recall correctly, bars are basically the same but involve the concentration of gas and dust near the center of the galaxy which generally has different dynamics from the disk out where we (and the spiral arms) live.

Hope this helps,

Ben